1. Field of the Invention
The present invention relates to an exhaust emission control device and method for an internal combustion engine, and an engine control unit, for purifying exhaust emissions by temporarily trapping NOx contained in exhaust gases discharged from the engine and performing reduction of the trapped NOx.
2. Description of the Related Art
Conventionally, there has been disclosed an exhaust emission control device for an internal combustion engine, e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2006-207487. This internal combustion engine is a diesel engine having an exhaust pipe, and a three-way catalyst and a NOx catalyst are arranged in the exhaust pipe in this order from the upstream side to the downstream side, for purifying HC and CO, i.e. reducing HC and CO emissions mainly during low temperature, and for purifying NOx, i.e. reducing NOx emissions, respectively. According to this exhaust emission control device, exhaust gases discharged from the engine during lean burn operation pass through the three-way catalyst and flow into the NOx catalyst to be temporarily trapped therein.
Then, when the amount of trapped NOx reaches a predetermined value, a rich spike operation is carried out in which the air-fuel ratio of a mixture supplied to the engine is controlled to a richer side than a stoichiometric air-fuel ratio. The rich spike operation causes unburned fuel components to be supplied to the NOx catalyst as reducing agent, whereby the NOx trapped in the NOx catalyst is reduced to be purified, i.e. changed into harmless ingredients. Further, the amount of reducing agent supplied to the NOx catalyst is calculated based on the air-fuel ratio detected by an air-fuel ratio sensor upstream of the three-way catalyst and the space velocity of exhaust gases, and when the cumulative value exceeds a threshold value set according to the amount of trapped NOx, it is judged that the reduction of NOx is complete, and the rich spike operation is terminated.
As described above, in the conventional exhaust emission control device, since the three-way catalyst is disposed upstream of the NOx catalyst, reducing agent supplied by the rich spike operation is consumed in the three-way catalyst to some extent, and then is supplied to the NOx catalyst. More specifically, reducing agent is oxidized and consumed by oxygen stored in the three-way catalyst during lean operation of the engine before the start of the rich spike operation, and accordingly, the amount of reducing agent supplied to the NOx catalyst is reduced.
However, in the conventional exhaust emission control device, the amount of reducing agent supplied to the NOx catalyst is only calculated based on the detected air-fuel ratio upstream of the three-way catalyst. Therefore, when the amount of reducing agent consumed in the three-way catalyst is large, the amount of reducing agent supplied to the NOx catalyst is calculated to be larger than the amount of actually supplied reducing agent, so that the cumulative value of the supply amount of reducing agent exceeds a threshold value before the reduction of NOx is completed, causing an increase in exhaust emissions due to insufficient reduction of NOx. Inversely, when the amount of reducing agent consumed in the three-way catalyst is small, the termination of the rich spike operation becomes later than it should be to supply an excessive amount of reducing agent, which increases exhaust emissions due to excessive supply of CO and HC, and degrades fuel economy.